The strength or amplitude of a received radio signal is typically measured by a device commonly known as a detector. In its simplest form, a detector may be a diode rectifier which converts an A.C. radio signal to a D.C. voltage proportional to the amplitude of the radio signal. In practice, the detector has a minimum signal level below which it will not produce a usable output. Accordingly, amplifiers are employed to generate a detectable signal. Another constraint is that the maximum detectable signal is limited both by the detector's breakdown voltage and an individual amplifier's saturation voltage. The detector's dynamic signal range is defined by these minimum and maximum signal levels.
To overcome limitations in the detector's dynamic range, logarithmic amplifiers are used. Logarithmic radio frequency (RF) amplifiers typically employ a chain of RF amplifiers of similar gain for cascade amplification of the input signal. Each amplifier provides an output that is a substantially linear function of the input signal until the input signal reaches a sufficient amplitude to saturate that amplifier. When this amplitude level is reached, the output of the amplifier remains constant at that limiting amplitude despite increases in the input signal level. Typically, if the signal level is very small, only by the final amplifier stage would the signal level have reached a detectable level. Conversely, if the signal level is large, the amplifiers will saturate in reverse order beginning with the last amplifier.
A significant problem with using a logarithmic amplifier chain having a large number of stages and a high total gain is the amplification of noise signals over a wide frequency range. In the absence of an input signal, internal amplifier noise could drive the later amplifier stages into saturation. In order to prevent premature saturation caused by wideband noise and to allow higher overall gain, the bandwidth must be restricted. One or more filters can be interposed between amplifier stages to limit the bandwidth so that only frequencies at or near the input signal frequency are amplified. In addition, to avoid instability caused by feedback between the input and output of a long amplifier chain, one or more frequency conversions may be required in the middle of such a chain in order to split the total desired amplification over different intermediate frequencies. The use of heterodyne mixers to effect frequency conversion may also involve bandwidth restricting filters to suppress unwanted frequency outputs or other spurious responses.
Unfortunately, the restriction of bandwidth necessarily introduces a time delay in the signal. For example, inserting a bandwidth limiting filter between first and second amplifier stages delays the output signal from the first stage to the second stage by a certain time period. Bandwidth restrictions in a logarithmic amplifier chain cause successive time delays in the responses of successive detector stages. Consequently, the sum of the detected outputs no longer represents the logarithm of the instantaneous signal amplitude.
Accordingly, it is desirable to provide a logarithmic amplifier/detector that overcomes the limitations of prior logarithmic amplifiers. Specifically, it is desirable to restrict the bandwidth of amplified signals and at the same time compensate for the resulting time delay in order to reduce transient distortion.